It is proposed to study the molecular basis of the interaction between collagen and platelets. The actual sites of recognition between the collagen molecule and the platelet membrane can be defined by assay of peptides obtained from collagen by CNBr cleavage and/or by tryptic and chrymotryptic digestion. All peptides will be assayed both for release and aggregation activities and for inhibition of these reactions. Once the active peptides are identified, sequence studies will be initiated to determine the amino acids that participate in the interaction with the platelet receptor site and collagen. This work should ultimately lead to synthesis of peptides that would either mimic the platelet activation of the whole collagen molecule or will inhibit this interaction. The total study has been initiated to isolate and identify the collagen receptor sites from membrane. This work has resulted in the isolation of a protein of a molecular weight of about 200,000 which upon incubation with collagen specifically inhibits collagen-platelet interaction. Preliminary results indicate that the forces involved in this interaction are both ionic and hydrophobic.